1. Field of the Invention
The present invention relates to a vehicle brake system capable of realizing a brake-by-wire in which a braking manipulation section is connected with a braking operation section by means of electric signals.
2. Discussion of the Related Art
In a so-called “brake-by-wire” wherein a braking manipulation section comprising a brake pedal and the like and a braking operation section comprising wheel brakes and the like are connected with each other by means of electric signals, the braking manipulation section is mechanically isolated from the braking operation section, and thus, it does not take place that unpleasant vibration is transmitted to the brake pedal when the brake force is varied upon anti-lock braking operation for example. Further, the brake-by-wire is advantageous to electric vehicles and hybrid vehicles which have a regenerative brake device, because the brake force by the wheel brakes can be varied to compensate for the variation in the regenerative brake force without giving the driver any uncomfortable feeling in manipulating the brake pedal even when the regenerative brake force is varied for some reason.
In a complete brake-by-wire, measures are taken to avoid the situation that the brake becomes unable to operate in the event of a failure in the electric system. For example, as described in Japanese unexamined, published patent application No. 2003-011808, there has been proposed a system wherein a master cylinder pressure is generated by a brake pedal and wherein the connection of a master cylinder to wheel brakes is blocked while the electric system is correctly operating. In the proposed system, however, when the electric system falls in failure, the connection of the master cylinder to the wheel brakes is completed, and for reduction in the probability that the electric power supply system breaks down completely, there are employed double electric power supply systems.
Further, United States patent application publication No. US2006/0163941 A1 (equivalent of Japanese international application publication No. 2005-532220) describes a brake-by-wire actuator for vehicle brake systems. In the actuator, a simulator operable by a brake pedal is provided, a signal from an actuation sensor provided in the simulator is transmitted to an electronic control unit for controlling a hydraulic pressure source in dependence on the signal, and the output from the hydraulic pressure source is connected to a brake force distribution device to operate wheel brakes. Further, means is provided for enabling the driver to operate the brakes with the driver's muscle power. In the brake actuator, in order that counterforce generated from the vehicle brake system is mechanically isolated not to be transmitted to a brake pedal in a by-wire mode, an idle space or distance is provided between the brake pedal or a member being in linkage to the same and an operating member which is connected at a downstream in a transmission train for a manipulation input.
Further, although being not classified in the brake-by-wire type, a vehicle brake device which is described in United States patent application publication No. US2005/0269875 A1 (equivalent of Japanese unexamined, published patent application No. 2005-349880) has been known as measures for realizing the cooperation with regenerative brake. The known vehicle brake device is provided with a hydraulic brake device, which is operable to increase the driver's braking manipulation force by a booster device in a predetermined boosting ratio, to generate a base hydraulic pressure depending on the increased braking manipulation force from a master cylinder connected to the booster device, to apply the generated base hydraulic pressure to wheel brakes (wheel cylinders) connected to the master cylinder through passages with hydraulic pressure control valves thereon to generate a base hydraulic brake force on respective wheels, and to supply the wheel brakes with a controlled hydraulic pressure which is built up by driving pumps so that a controlled hydraulic brake force can be applied to the respective wheels.
Further, the device of the last mentioned United States publication is provided with a regenerative brake device, which, by applying an allocated regenerative brake force to some wheels, generates a target brake force corresponding to a braking manipulation force in combination with the base hydraulic brake force, and variation detection, means for detecting the variation of the regenerative brake force being actually generated by the regenerative brake device, from the allocated regenerative brake force. When a substantial variation is detected by the variation detection means, a controlled hydraulic pressure is generated by driving the pumps of the hydraulic brake device and by controlling the hydraulic pressure control valves to generate a controlled hydraulic brake force on the vehicle wheels in dependence on the controlled hydraulic pressure, so that a deficiency of the regenerative force can be compensated for the variation detected by the variation detection means.
However, in the vehicle brake system described in the first mentioned Japanese application, the reliability in avoiding the complete failure of the electrical system is enhanced, but an increase in cost caused by duplicating the power supply system is unavoidable.
In the vehicle brake system described in the second mentioned United States publication, the manipulation of the brake pedal is mechanically transmitted all the time to the wheel brakes without depending on electric signals, and hence, a brake force approximately equal to that in the properly operating state can be secured even in the event of the malfunction of the electric system. On the contrary, however, the brake-by-wire actuator should be mounted at a position where a vacuum booster device and a master cylinder would otherwise be mounted in conventional vehicles and to which a manipulation force from the brake pedal would be applied, and further, there is need a device such as, e.g., ABS or the like which controls the vehicle brakes individually. Therefore, great restraints are imposed on the cost as well as on the design for installation in the vehicle.
In the vehicle brake device described in the last mentioned United States publication, the manipulation of the brake pedal is mechanically transmitted all the time to the wheel brakes, and hence, the reliability against the breakdown of the electric system is enhanced. In the device, on the contrary, the regenerative brake force cannot bear the entirety of the brake force required for the vehicle. Further, where the regenerative brake force varies in the course of a braking operation, the total brake force which depends on the braking manipulation force being exerted on the brake pedal can be maintained invariable by adjusting the controlled hydraulic brake force to compensate for the variation in the regenerative brake force. Adjusting the controlled hydraulic brake force involves adjusting the operation of the pumps which generate a controlled hydraulic pressure for the controlled hydraulic brake force, and this results in consuming the brake fluid from the master cylinder by the pumps, so that the variation in the stroke of the brake pedal is unavoidable. Where the regenerative brake force is increased, the variation in the brake pedal stroke becomes greater, and this may cause the driver to have an unpleasant feeling. These problems arise as a natural consequence from the fact that the brake-by-wire is not realized in the vehicle brake device described in the last mentioned United States publication.